Apparatus and method using near infrared reflectometry to reduce the effect of positional changes during spinal cord stimulation

ABSTRACT

A positionally sensitive spinal cord stimulation apparatus and method using near-infrared (NIR) reflectometry are provided for automatic adjustments of spinal cord stimulation. The system comprises an electrode assembly with an integrated optical fiber sensor for sensing spinal cord position. The integrated optical fiber sensor, comprising a pair of optical elements for emitting light from an IR emitter and for collecting reflected light into a photodetector, determines a set of measured photocurrents. As the spinal cord changes position, the angles of incidence for light from the IR emitter and the measured optical intensities change. Electrode pulse characteristics are adjusted in real time, based on the set of measured optical intensities, to minimize changes in stimulation perceived by the patient during motion. The system includes automatic calibration of the optical fiber sensor when the patient is at rest, and a patient orientation detection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 14/336,796, filed Jul. 21, 2014, which is aContinuation-in-Part Application of U.S. patent application Ser. No.14/019,240, filed Sep. 5, 2013, which is a Continuation-in-PartApplication of U.S. patent application Ser. No. 13/780,470, filed Feb.28, 2013, now U.S. Pat. No. 9,132,273, which is a Continuation-in-PartApplication of U.S. patent application Ser. No. 13/567,966, filed Aug.6, 2012, now U.S. Pat. No. 8,543,213, which is a Continuation of U.S.patent application Ser. No. 12/925,231, filed Oct. 14, 2010, now U.S.Pat. No. 8,239,038. U.S. patent application Ser. No. 14/019,240 claimspriority to U.S. Provisional Patent Application No. 61/867,413, filedAug. 19, 2013. Each patent application identified above is incorporatedhere by reference in its entirety to provide continuity of disclosure.

FIELD OF DISCLOSURE

This disclosure relates generally to spinal cord stimulation (SCS) andtechnique for automatic adjustments of SCS using near-infrared (NIR)reflectometry.

BACKGROUND

Spinal cord stimulation is a technique which uses an implanted electrodearray to control chronic pain. The electrode array is typicallyimplanted in a fixed position within the epidural space near the spinalcord. A signal generator delivers current pulses to the spinal cord viathe implanted electrode array. The current pulses help block theperception of pain.

In FIG. 1, spinal column 1 is shown to have a number of vertebrae,categorized into four sections or types: lumbar vertebrae 2, thoracicvertebrae 3, cervical vertebrae 4 and sacral vertebrae 5. Cervicalvertebrae 4 include the 1st cervical vertebra (C1) through the 7thcervical vertebra (C7). Just below the 7th cervical vertebra is thefirst of twelve thoracic vertebrae 3 including the 1st thoracic vertebra(T1) through the 12th thoracic vertebra (T12). Just below the 12ththoracic vertebrae 3, are five lumbar vertebrae 2 including the 1stlumbar vertebra (L1) through the 5th lumbar vertebra (L5), the 5thlumbar vertebra being attached to sacral vertebrae 5 (S1 to S5), sacralvertebrae 5 being naturally fused together in the adult.

In FIG. 2, representative vertebra 10, a thoracic vertebra, is shown tohave a number of notable features which are in general shared withlumbar vertebrae 2 and cervical vertebrae 4. The thick oval segment ofbone forming the anterior aspect of vertebra 10 is vertebral body 12.Vertebral body 12 is attached to bony vertebral arch 13 through whichspinal nerves 11 run. Vertebral arch 13, forming the posterior ofvertebra 10, is comprised of two pedicles 14, which are short stoutprocesses that extend from the sides of vertebral body 12 and bilaterallaminae 15. The broad flat plates that project from pedicles 14 join ina triangle to form a hollow archway, spinal canal 16. Spinous process 17protrudes from the junction of bilateral laminae 15. Transverseprocesses 18 project from the junction of pedicles 14 and bilaterallaminae 15. The structures of the vertebral arch protect spinal cord 20and spinal nerves 11 that run through the spinal canal.

Surrounding spinal cord 20 is dura 21 that contains cerebrospinal fluid(CSF) 22. Epidural space 24 is the space within the spinal canal lyingoutside the dura.

Referring to FIGS. 1, 2 and 3, the placement of an electrode array forspinal cord stimulation according to the prior art is shown. Electrodearray 30 is positioned in epidural space 24 between dura 21 and thewalls of spinal canal 16 towards the dorsal aspect of the spinal canalnearest bilateral laminae 15 and spinous process 17.

FIG. 4 shows a prior art electrode array 30 including electrode contacts35 sealed into elastomeric housing 36. Electrode array 30 has electrodeleads 31 which are connected to electrical pulse generator 32 andcontroller 33. The electrical pulse generator may be outside of the bodyor it may be implanted subcutaneously. Each electrode contact has aseparate electrical conductor in electrode leads 31 so that the currentto each contact may be independently controlled.

The anatomical distribution of parasthesias is dependent upon thespatial relationship between a stimulating electric field generated bythe electrode array and the neuronal pathways within the spinal cord.The distribution may be changed by altering the current across one ormore electrodes of the electrode array. Changing anode and cathodeconfigurations of the electrode array also alters the distribution andhence, the anatomical pattern of the induced parasthesias.

Proper intensity of the current pulses is important. Excessive currentproduces an uncomfortable sensation. Insufficient current producesinadequate pain relief. Body motion, particularly bending and twisting,causes undesired and uncomfortable changes in stimulation due to motionof the spinal cord relative to the implanted electrode array.

There are methods and systems for controlling implanted devices withinthe human body. For example, Ecker et al, in U.S. Patent Publication No.2010/0105997, discloses an implantable medical device that includes acontroller and a plurality of sensor modules. A sensor includes at leastone light source that emits light at a particular wavelength, whichscatters through blood-perfused tissue a detector senses the lightreflected by blood mass of a patient.

U.S. Pat. No. 7,684,869 to Bradley, et al. discloses a system using aninterelectrode impedance to determine the relative orientation of a leadwith respect to other leads in the spinal column. Bradley et al. furtherdisclose that interelectrode impedance may be used to adjust stimulationenergy.

U.S. Patent Publication No. 2009/0118787 to Moffitt, et al. discloseselectrical energy conveyed between electrodes to create a stimulationregion. Physiological information from the patient is acquired andanalyzed to locate a locus of the stimulation region. The stimulationregion is electronically displaced.

U.S. Pat. No. 7,413,474 to Liu, et al. discloses carbon nano-tubecomposites (see, for example, abstract, FIG. 2 and col. 3:11. 21-35).The disclosure of U.S. Pat. No. 7,413,474 is incorporated herein byreference.

Deficiencies exist in the prior art related to accuracy of spinal cordstimulation in relieving pain under changing circumstances. Thedeficiencies are most pronounced while the patient is moving. The priorart does not provide a satisfactory way to automatically adjust spinalcord stimulation to compensate for motion between the electrodes and thespinal cord to maintain a constant level of pain relief during patientmotion.

SUMMARY

Embodiments of the present disclosure operate to automatically adjustspinal cord stimulation to compensate for patient movement. Automaticadjustment results in consistent parasthesias and conservation ofbattery power.

The disclosure demonstrates a novel optical sensor, generally useful inmany fields of endeavor, in which a probe light beam is emitted from afirst optical element and a responsive light beam is collected by asecond optical element. In a preferred embodiment, the first opticalelement is coupled to the end of a first optical fiber and the secondoptical element is coupled to the end of a second optical fiber. Thefirst optical fiber is further coupled to an active optical source. Thesecond optical fiber is further coupled to an active optical detector.

Disclosed is a stimulator system having a surgical lead encasing thefirst and second optical fibers, electrode contacts and a controller.The optical source, operatively connected to the controller, generatesan emitted light beam into the first optical fiber. The opticaldetector, also operatively connected to the controller, receivesreflected light beams from the second optical fiber. Electrodes areoperatively connected to the controller and the controller directscurrents to the electrodes based on the reflected light beams.

In an aspect of the system, the reflected light beams are derived fromthe probe light beam as it interacts with the spinal cord of a hostpatient. In another aspect, the distance from surgical lead to thespinal cord is determined using optical reflectometry.

In another aspect of the system, the controller derives current pulseparameters for currents based on time averaging current pulsefrequencies, time averaging current amplitudes, time averaging currentpulse-widths, interpolating current pulse frequencies, interpolatingcurrent amplitudes and interpolating current pulse-widths.

In another aspect of the system, the controller includes an orientationdetector and derives a real-time position of a host patient.

In a preferred embodiment, the system further comprises a calibrationand programming unit operatively connected to the controller forcalibrating the current pulse amplitudes, pulse widths and pulsefrequencies. The current pulse amplitudes for the electrodes arecalibrated to photocurrents derived from the optical detector while thepatient is placed in different positions. Current pulse amplitude andvalues of photocurrents are stored in a calibration table correspondingpatient position.

In another aspect, the controller is programmed to detect patient motionfrom photocurrents. When no motion has occurred for a predetermined timeperiod, the controller recalibrates the optical source.

In another aspect the controller is programmed to detect patientorientation using an orientation sensor. When no change in orientationhas occurred for a time period, the controller recalibrates the opticalsource.

BRIEF DESCRIPTION OF DRAWINGS

The following disclosure is understood best in association with theaccompanying figures. Like components share like numbers.

FIG. 1 shows a view of the human spine showing the various types ofvertebrae and an approximate position of an electrode array for spinalcord stimulation.

FIG. 2 shows an axial view of a thoracic vertebra indicating theposition of the spinal cord and an electrode array for spinal cordstimulation.

FIG. 3 shows a sagittal cross-sectional view of the human spine showingthe approximate position of an electrode array for spinal cordstimulation.

FIG. 4 shows a prior art electrode array and a lead connector for spinalcord stimulation.

FIG. 5a shows a preferred embodiment of a surgical lead cable.

FIG. 5b shows a preferred embodiment of a surgical lead cable.

FIG. 5c is a cross-sectional view of a preferred embodiment of aconnector.

FIG. 6 shows a preferred placement of a preferred surgical lead in thespinal column.

FIG. 7 shows a cross-sectional sagittal view of a surgical lead.

FIG. 8a shows a cross-sectional axial view of a surgical lead with aspinal cord at a forward position.

FIG. 8b shows a cross-sectional axial view of a surgical lead with aspinal cord at a rightward position.

FIG. 8c shows a cross-sectional axial view of a surgical lead with aspinal cord at backward position.

FIG. 8d shows a cross-sectional axial view of a surgical lead with aspinal cord at leftward position.

FIG. 9 shows the relative electric field produced by a preferredembodiment for the spinal cord in various positions within the spinalcanal.

FIG. 10 shows a preferred embodiment of a surgical lead.

FIG. 11 shows a preferred placement of a surgical lead in a spinalcolumn.

FIG. 12 shows a cross-sectional axial view of a surgical lead.

FIG. 13a shows a cross-sectional axial view of a surgical lead locatedin relation to a spinal cord at a forward position.

FIG. 13b shows a cross-sectional axial view of a surgical lead locatedin relation to a spinal cord at a rightward position.

FIG. 13c shows a cross-sectional axial view of a surgical lead locatedin relation to a spinal cord at a backward position.

FIG. 13d shows a cross-sectional top-view of a surgical lead located inrelation to a spinal cord at a leftward position.

FIG. 14 shows a perspective view of a preferred embodiment of a pairedpercutaneous lead.

FIG. 15a is a cross-sectional view of an embodiment of a percutaneouslead.

FIG. 15b is a cross-sectional view of a preferred embodiment of asurgical lead.

FIG. 16 shows preferred placement of a paired percutaneous surgical leadin a spinal column.

FIG. 17 shows a cross-sectional axial view of a pair of percutaneousleads near a spinal cord.

FIG. 18a shows a cross-sectional axial view of a paired percutaneoussurgical lead located in relation to a spinal cord at a forwardposition.

FIG. 18b shows a cross-sectional axial view of a paired percutaneoussurgical lead located in relation to a spinal cord at a rightwardposition.

FIG. 18c shows a cross-sectional axial view of a paired percutaneoussurgical lead located in relation to a spinal cord at a backwardposition.

FIG. 18d shows a cross-sectional axial view of a paired percutaneoussurgical lead located in relation to a spinal cord at a leftwardposition.

FIG. 19 is a block diagram of a preferred embodiment of a stimulatorsystem.

FIG. 20 is a block diagram of a preferred embodiment of a pulsegenerator and signal processing unit.

FIG. 21 is a block diagram of the components of a preferred embodimentof an SCS controller.

FIG. 22 is a block diagram of the components of a preferred embodimentof a calibration and programming unit.

FIG. 23 is a state diagram of a preferred embodiment of a stimulatorcontrol system.

FIG. 24 is a graphic representation of a preferred embodiment of acalibration table.

FIG. 25 is a graphic representation of a preferred embodiment of acalibration table.

FIG. 26 is a flow chart of a method of operation for a stimulatorsystem.

FIG. 27a is a flow chart of a method of performing a stimulationroutine.

FIG. 27b is a flow chart of an alternate method of performing astimulation routine.

FIG. 28a is a flow chart of a method for calibrating an optical source.

FIG. 28b is a flow chart of an alternate method for calibrating anoptical source.

FIG. 29a is a flow chart of a method of calibration of electrode pulsesimulation amplitude.

FIG. 29b is a flow chart of an alternate method of calibration ofelectrode pulse simulation amplitude.

FIG. 30a is a flow chart of an alternate method of performing astimulation routine.

FIG. 30b is a flow chart of a method of adjusting cycle time andelectrode pulse stimulation current.

FIG. 30c is a flow chart of a method to accelerate calibration of anoptical source.

DETAILED DESCRIPTION

The distance between a stimulating electrode and the spinal cord surfacemay be inferred from a function dependent upon: 1) the optical pathlengths of light between a near infrared light emitter and detectors,where the light is reflected from the spinal cord; 2) the spinal cordgeometry; 3) the optical divergence of the light emitter; and 4) thepresence of chromophores in the optical path.

The dura surrounding the spinal cord itself is translucent to nearinfrared light. Near infrared light will be scattered by, and willreflect from, the spinal cord. Cerebrospinal fluid (CSF) will negligiblyscatter near infrared light and will not act as a significant reflectorof near-infrared light. Light from the light emitter passes through thethin, relatively avascular dura to enter the CSF. Light incident on thespinal cord experiences scatter resulting in a portion being reflectedand another portion being absorbed by chromophores.

Optical absorption in a fluid medium may be described by theBeer-Lambert Law (Beer's Law), which is reasonably accurate for a rangeof chromophores and concentrations. Beer's Law states that the opticalabsorbance of a fluid with a chromophore concentration varies linearlywith path length through the fluid and the chromophore concentration as:A_(λ)=ε_(λ)bc,  (Eq. 1)where:

-   -   ε_(λ)=molar absorptivity or extinction coefficient of the        chromophore at wavelength λ (the optical density of a 1-cm thick        sample of a 1 M solution);    -   b=sample path length in centimeters; and,    -   c=concentration of the compound in the sample, in molarity (mol        L⁻¹).

The absorbance (A_(λ)) at a wavelength λ is related to the ratio oflight energy passing through the fluid, I, to the incident light energy,I₀, inA _(λ)=−log (I/I ₀).   (Eq. 2)

For deoxyhemoglobin and oxyhemoglobin, the extinction coefficientspectra are well known.

The path length within the spinal cord is dependent upon the geometry ofthe ellipsoid shaped spinal cord cross-section and its normal vectorrelative to the optical axes of the emitter and detector pair.

The optical path length within CSF is roughly equal to the nominalgeometric path length as the scatter is small and the index ofrefraction does not vary considerably along the path. Light absorptionof the CSF may be approximated by that of its primary constituent, H₂O.Sensitivity of the system to CSF path length may be optimized using alight wavelength at a local maxima of the water extinction coefficientcurve near 950-1100 nm.

When considering the light emitter wavelength, one must also considerthe extinction coefficients of the primary chromophores, deoxy- andoxy-hemoglobin. To minimize effects of blood flow changes within thespinal cord (although these are thought to be insignificant in thequasi-static sense), one may select the isosbestic wavelength of thesechromophore species, preferably at about 805 nm.

The geometry of the light emitter and detector aperture relative to thespinal cord is the parameter most prone to variability. The varianceresults from factors such as dependence upon placement of the electrodewithin the spinal canal, canal diameter, spinal cord shape, spinal cordcaliber, and presence of scoliotic or kyphotic curvature within thespine. Consequently, this geometric parameter is the primary reason thatthe system must be calibrated, in situ, in vivo. Spinal cord positionmay then be inferred through various methods from data obtained atordinal body positions.

The effects of geometry may be minimized by minimizing the angle betweenthe light emitter and optical detector optical axes relative to thespinal cord surface normal vector.

The beam divergence of the light emitter relative to the incident andreflected rays will influence the detected light amplitude.

It is desirable to maintain a constant electric field at a group oftarget cells in the spinal cord as the spinal cord moves in order toconsistently reduce the transmission of a pain sensation to the brain.With the patient in a prone position or bending forward (0° direction),the spinal cord moves anterior within its orbit in the spinal canal. Anincrease in stimulation pulse amplitude for each electrode pair isrequired to maintain the same electric field density. In the rightlateral position or bent to the right (90° direction), the spinal cordmoves to the right within its orbit in the spinal canal. A decrease inelectrode stimulation pulse amplitude in the right electrode and anincrease in electrode stimulation pulse amplitude in the left electrodeof the electrode pair is required. In the supine position or bendingbackward (180° direction), the spinal cord moves dorsally within itsorbit within the spinal canal. A decrease in electrode stimulation pulseamplitude bilaterally is required to maintain a constant electric fieldacross the spinal cord. In the left lateral position or bent toward theleft (270° direction), the spinal cord moves to the left within itsorbit. A decrease in electrode stimulation pulse amplitude in the leftelectrode and an increase in electrode stimulation pulse amplitude inthe right electrode of the electrode pair is required.

Referring to FIG. 5a , a preferred embodiment of surgical lead 520 isshown. Surgical lead 520 includes an elastomeric housing 501 connectedto lead 510 and to lead 511. Optical fiber 502, optical fiber 503,electrodes 512 and electrodes 513 are embedded in elastomeric housing501. In a preferred embodiment, the elastomeric housing is generallyrectangular. Other shapes may suffice. Optical fiber 502 is terminatedwith optical element 509. Optical fiber 503 is terminated with opticalelement 508. Lead 510 encloses optical fiber 502 and wires 504 and isterminated with opto-electrical connector 506. Lead 511 encloses opticalfiber 503 and wires 505 and is terminated with opto-electrical connector507.

FIG. 5b shows a cross-sectional view of lead 510. Leads 510 and 511 areidentical in structure. Lead 510 includes outer surface 515 whichencapsulates wires 504, lumen 516 and filler material 519. Lumen 516encloses optical fiber 502. Outer surface 515 is comprised of a shieldfor electromagnetic signals. In a preferred embodiment, the outersurface is made of a conductive material including metal sheeting, wiremesh and metal coatings. Filler material 519 is comprised of a polyimidepolymer. In an alternate embodiment, filler material 519 can includeadditional materials with physical properties that enhanceelectromagnetic shielding properties such as conductive particles and/orcarbon nano-tube composites.

Referring to FIG. 5c , opto-electrical coupler 506 is shown.Opto-electrical couplers 506 and 507 are identical in structure. In apreferred embodiment, opto-electrical coupler 506 includes case 560 withepoxy header 552, and optical fiber with cladding 553. Epoxy header 552includes cavity 557. Cavity 557 includes spring loaded connectors 556which are electrically connected to the pulse generator and sender unit(which will be further described). Case 560 includes cavity 561connecting to cavity 557 and terminating in NIR transparent window 562.In a preferred embodiment, NIR transparent window 562 is flat. However,in an alternate embodiment, NIR transparent window is a lens. In anotherexample, collimating lens 554 is an optical fiber with a polished end.Opto-electrical component 565 is situated behind the NIR transparentwindow. NIR transparent window 562 serves as a hermetic barrier betweencavity 561 and opto-electrical component 565. NIR transparent window 562also serves as an optical coupler. Opto-electrical component 565 may bean optical emitter. Opto-electrical component 565 may be an opticaldetector. In use, the lead is inserted into cavity 557 during a surgicalprocedure until collimating lens 554 is directly adjacent barrier 562.The spring connectors override and engage contacts 551 on lead 550.

FIG. 6 shows a cross-sectional view of a vertebra 622 and spinal cord625. Surgical lead 620 is implanted in epidural space 626 of vertebra622 between the dura 621 and the walls of the spinal canal 629 using asurgical procedure.

Referring to FIG. 7, a sagittal view of surgical lead 620 is shownimplanted in relation to dura 721 and spinal cord 720. Spinal cord 720includes target cells 719. Surgical lead 620 is implanted outside dura721, approximately aligned with midline axis 724.

In use, probe light beam 761 is transmitted through optical fiber 611and emitted from optical element 608. The probe light beam propagatesthrough spinal canal, experiences absorption by the dura and the spinalfluid, and is reflected and scattered by the spinal cord. Reflectedlight beam 762 is collected by optical element 609 and is transmittedthrough optical fiber 613. Electrodes 512 and 513 supply stimulationcurrent to the spinal cord based on the intensity of the reflected lightbeam.

Referring to FIGS. 8a-8d , axial views of the spinal cord 820 andsurgical lead 800 are shown with spinal cord 820 and dura 821 in variouspositions in the spinal canal caused by movement of the patient. Thefigures are shown in relation to coronal axis 824 and sagittal axis 825.

Referring to FIG. 8a , spinal cord 820 is in a forward position toward0° along sagittal axis 825. Path P₁ defines a light path from opticalelement 608 to reflection point R₁ and then to optical element 609. Thelength of path P₁ is D₁. Optical element 608 emits light from opticalsource 805 along path P₁ where it is reflected at point R₁ by the spinalcord surface. Optical element 609 collects light from path P₁ afterreflection at point R₁. Light collected by optical element 609, isdetected by photodetector 806 and converted to photocurrent I₁ inresponse.

In FIG. 8b , spinal cord 820 is in a rightward position, rotated byangle 828 from sagittal axis 825 where target cells 819 are shiftedrightward toward 90° and parallel to coronal axis 824 by distance 827.Path P₂ defines a light path from optical element 608 to reflectionpoint R₂ and then to optical element 609. The length of path P₂ is D₂(which is less than D₁). Optical element 609 emits light from opticalemitter 805 along path P₂ where it is reflected at point R₂ by thespinal cord surface. Optical element 608 collects light from path P₂after reflection at point R₂. Light collected by optical element 608, isdetected by photodetector 806 and converted to photocurrent I₂ inresponse.

In FIG. 8c , spinal cord 820 is in a posterior position shifted by adistance 826 towards optical elements 608 and 609 along sagittal axis825. Path P₃ defines a light path from optical element 609 to reflectionpoint R₃ and then to optical element 608. The length of path P₃ is D₃(which is less than D₁ or D₂). Optical element 609 emits light fromoptical emitter 805 along path P₃ where it is reflected at point R₃ bythe spinal cord surface. Optical element 608 collects light from path P₃after reflection at point R₃. Light collected by optical element 608, isdetected by photodetector 806 and converted to photocurrent I₃ inresponse.

In FIG. 8d , spinal cord 820 is in a left position, rotated by angle 830from sagittal axis 825 where target cells 819 are shifted leftward alongsagittal axis 824 by distance 829. Path P₄ defines a light path fromoptical element 609 to reflection point R₄ and then to optical element608. The length of path P₄ is D₄ which is less than D₁, but about thesame as D₂. Optical element 609 emits light from optical emitter 805along path P₄ where it is reflected at point R₄ by the spinal cordsurface. Optical element 608 collects light from path P₄ afterreflection at point R₄. Light collected by optical element 608, isdetected by photodetector 806 and converted to photocurrent I₄ inresponse.

An electric field produced by the electrodes 801, including electrodes812 and electrodes 813, stimulates target cells 819 in the spinal cord820. Current amplitude is supplied to the electrodes in pulses, eachhaving a pulse width and a pulse frequency. The relative currentamplitude must be increased as the target cells move away from theelectrodes. Also, the intensity of the reflected signal decreases as thesurface of the spinal cord moves away from the optical elements. Hence,as the reflected light beam decreases, the current amplitude mustincrease to maintain the same electrical field intensity at the targetcells.

FIG. 9 shows a plot 900 of relative electric field strength required tobe generated at the electrodes in order to maintain a constantelectrical field at target cells 819, as the spinal cord is movedthrough an orbit of 360° in the spinal canal.

The foregoing results are tabulated in Table 1.

TABLE 1 Stimulation Photodetector Current Position Current, I Amplitude,A 1. Front 0⁰ Low High 2. Right 90° Medium Medium 3. Back 180° High Low4. Left 270° Medium Medium

Referring to FIG. 10, an alternate embodiment of a surgical lead isshown. Surgical lead 1000 includes an elastomeric housing 1001 connectedto lead 1010 and to lead 1011. Embedded in elastomeric housing 1001, areoptical fiber 1002, optical fiber 1003, electrodes 1012 and electrodes1013. Optical fiber 1002 is terminated with an optical element 1008.Optical fiber 1003 is terminated with optical element 1009.

Lead 1010 encloses optical fiber 1002 and wires 1004 which areterminated in opto-electrical connector 1006. Lead 1011 encloses opticalfiber 1003 and wires 1005 which are terminated in opto-electricalconnector 1007.

Referring to FIG. 11, a cross-sectional view of vertebra 1122 is shownenclosing spinal cord 1125. Surgical lead 1100 is placed in the epiduralspace 1126 of vertebra 1122 between dura 1121 and the walls of thespinal canal 1129. Surgical lead 1100 includes optical elements 1108 and1109.

Referring to FIG. 12, a top view of surgical lead 1200 is shownimplanted adjacent dura 1221. Optical source 1205 and optical detector1204 are shown schematically. Surgical lead 1200 includes opticalelement 1208 coupled to optical source 1205 and optical element 1209coupled to optical detector 1204. Surgical lead 1200 is positionedwithin an operational range of target cells 1219.

In use, light beam 1261 is emitted from optical source 1205, propagatesthrough optical fiber 1203 and exits from optical element 1208. Thelight beam then propagates through spinal canal, experiences absorptionby the dura and the spinal fluid, and is reflected and scattered by thesurface of the spinal cord. Reflected light beam 1262 is collected byoptical element 1209. Reflected light beam 1262 propagates throughoptical fiber 1202 and is detected by optical detector 1204.

Referring to FIGS. 13a-13d , top views of spinal cord 1220 and surgicallead 1200 are shown with spinal cord 1220 in various positions.Electrodes 1212 and 1213 supply stimulation current to the spinal cord.Surgical lead 1200 is approximately aligned with coronal axis 1324. Thefigures are shown in relation to coronal axis 1324 and sagittal axis1325.

Referring to FIG. 13a , the spinal cord is positioned forward. Path P₅defines a light path from optical element 1208 to reflection point R₅and then to optical element 1209. The length of path P₅ is D₅. Opticalelement 1208 emits light along path P₅ and optical element 1209 collectslight from path P₅ after reflection at point R₅ from spinal cord 1220.Light collected by optical element 1209 is detected by photodetector1204 which produces a photocurrent of I₅ in response.

Referring to FIG. 13b , the spinal cord is rotated through angle 1328and positioned rightward by a distance 1327. Path P₆ defines a lightpath from optical element 1208 to reflection point R₆ and then tooptical element 1209. The length of path P₆ is D₆ which is less than thelength D₅. Optical element 1208 emits light along path P₆ and opticalelement 1209 collects light from path P₆ after reflection at point R₆from spinal cord 1220. Light collected by optical element 1209 isdetected by photodetector 1204 which produces a photocurrent of I₆ inresponse. I₆ is greater than I₅.

Referring to FIG. 13c , the spinal cord is positioned towards the backand displaced by a distance 1326. Path P₇ defines a light path fromoptical element 1208 to reflection point R₇ and then to optical element1209. The length of path P₇ is D₇ which is shorter than length D₅ or D₆.Optical element 1208 emits light along path P₇ and optical element 1209collects light from path P₇ after reflection at point R₇ from spinalcord 1220. Light collected by optical element 1209 is detected byphotodetector 1204 which produces a photocurrent of I₇ in response. I₇is greater than I₅ and I₆.

Referring to FIG. 13d , the spinal cord is rotated through angle 1330and positioned leftward by a distance 1329. Path P₈ defines a light pathfrom optical element 1208 to reflection point R₈ and then to opticalelement 1209. The length of path P₈ is D₈ which is less than length D₅but about the same as D₆. Optical element 1208 emits light along path P₈and optical element 1209 collects light from path P₈ after reflection atpoint R₈ from spinal cord 1220. Light collected by optical element 1209is detected by photodetector 1204 which produces a photocurrent of I₈ inresponse. I₈ is about the same as I₆.

An electric field produced by electrodes 1012 and electrodes 1013,stimulates target cells 1219 in the spinal cord 1220. Table 1 indicatesthe relative levels of electrode stimulation current required based onphotocurrent.

Referring to FIG. 14, alternate embodiment 1450 is shown in which twopercutaneous leads are provided. Percutaneous lead 1461 includes opticalfiber 1451, optical element 1459, electrodes 1471 and contacts 1475.Optical fiber 1451 is coupled to optical element 1459. Percutaneous lead1461 also includes electrical wires (not shown). The percutaneous leadterminates in opto-electrical connector 1453.

Percutaneous lead 1462 includes optical fiber 1452, optical element1458, electrodes 1472 and contacts 1476. Optical fiber 1452 is coupledto optical element 1458. Percutaneous lead 1462 also includes electricalwires (not shown). The percutaneous lead terminates in opto-electricalconnector 1454. Percutaneous lead 1461 is identical to percutaneous lead1462.

Referring to FIG. 15a , a preferred embodiment of a percutaneous lead isshown. Percutaneous lead 1500 includes lead body 1501 in which anoptical fiber 1510 is embedded. Optical fiber 1510 is coupled tocollimating lens 1504. Lead body 1501 also includes electrodes 1507connected by electrical wires 1509 to contacts 1508. Optical fiber 1510includes a cladding 1502 and a core 1503 co-centered on fiber optic axis1548. Optical fiber 1510 is coupled to an angled lens assembly 1505.

Angled lens assembly 1505 includes a housing 1549 coupled to opticalfiber 1510 and core 1503. Housing 1549 further includes collimating lens1542 and reflective surface 1544 at an angle α from fiber optic axis1548. Collimating lens 1542 and reflective surface 1544 are positionedto collimate light along axis 1547. Angle α is preferably in the rangeof about 30° to about 60°.

Referring to FIG. 15b , an alternate embodiment of a percutaneous leadis shown. Percutaneous Lead 1520 includes lead body 1521 in which anoptical fiber 1530 is embedded. Optical fiber 1530 is coupled tocollimating lens 1524. Lead body 1521 also includes electrodes 1527connected by electrical wires 1529 to contacts 1528. Optical fiber 1530includes cladding 1522 and core 1523. Optical fiber 1530 includesnegative axicon 1525. For an uncoated negative axicon, angular extent βis less than about 33° for typical glass. The maximum value of β isdetermined as the complement of the critical angle χ for the opticalmaterial in core 1523. The complement of the critical angle is (90°−χ).If the negative axicon has a reflective coating then angular extent β isapproximately 45°.

Referring to FIG. 16, a cross-sectional view of vertebra 1622 is shownenclosing spinal cord 1620. Percutaneous lead 1661 and percutaneous lead1662 are implanted in epidural space 1626 of vertebra 1622 between dura1621 and the walls of the spinal canal 1629. In a preferred embodiment,the percutaneous leads are implanted side-by-side at a predetermineddistance apart, adjacent, and generally parallel to, each other.Placement of percutaneous leads 1661 and 1662 can be accomplishedthrough insertion of the leads through needles placed percutaneouslyinto the epidural space.

Referring to FIG. 17, a cross-sectional axial view of the percutaneousleads implanted is shown. Percutaneous lead 1761 includes opticalelement 1708 and electrodes 1471. Optical element 1708 is coupled to anoptical source 1705. Percutaneous lead 1762, also implanted outside dura1721, includes optical element 1709 and electrodes 1472 where opticalelement 1709 is coupled to optical detector 1704. Percutaneous leads1761 and 1762 are positioned within an operational range of target cells1719 of spinal cord 1720.

In use, a light beam is emitted from optical source 1704, propagatesthrough optical fiber 1703 and exits from optical element 1708 as lightbeam 1781. Light beam 1781 propagates through the spinal canal,experiences absorption by the dura and the spinal fluid, and isreflected and scattered to create reflected light beam 1782. Reflectedlight beam 1782 is collected by optical element 1709 and detected byoptical source 1705.

Referring to FIGS. 18a-18d , spinal cord 1720 is shown in variouspositions in the spinal canal in relation to coronal axis 1824 andsagittal axis 1825.

Referring to FIG. 18a , the spinal cord is positioned forward, path P₉defines a light path from optical element 1708 to reflection point R₉and then to optical element 1709. Optical element 1708 emits light,along path P₉. Optical element 1709 collects light after reflection frompoint R₉. Light collected by optical element 1709 is detected by opticalsource 1705 which produces a photocurrent I₉ in response.

Referring to FIG. 18b , the spinal cord is rotated through angle 1828and positioned rightward by a distance 1827 towards 90°. Path P₁₀defines a light path from optical element 1708 to reflection point R₁₀and then to optical element 1709. The length of path P₁₀ is less thanthe length of path P₉. Optical element 1708 emits light, including lightalong path P₁₀. Optical element 1709 collects light after reflection atpoint R₁₀. Reflected light collected by optical element 1708 is detectedby photodetector 1705 which produces a photocurrent I₁₀ in response. I₁₀is greater than I₉.

Referring to FIG. 18c , the spinal cord is positioned towards the backand displaced dorsally by a distance 1826. Path P₁₁ defines a light pathfrom optical element 1708 to reflection point R₁₁ and then to opticalelement 1709. The length of path P₁₁ is shorter than the length of pathsP₉ or P₁₀. Optical element 1708 emits light, including light along pathP₁₁. Optical element 1709 collects reflected light. Reflected lightcollected by optical element 1709 is detected by photodetector 1705which produces a photocurrent I₁₁ in response. I₁₁ is greater than I₉and I₁₀.

Referring to FIG. 18d , the spinal cord is rotated through angle 1830and positioned leftward by a distance 1829 towards 270°. Path P₁₂defines a light path from optical element 1708 to reflection point R₁₂and then to optical element 1709. The length of path P₁₂ is less thanlength of path P₉ but about the same as the length of path P₁₀. Opticalelement 1708 emits light, including light along path P₁₂. Opticalelement 1709 collects reflected light, including light from path P₁₂.Reflected light collected by optical element 1709 is detected byphotodetector 1705 which produces a photocurrent I₁₂ in response. I₁₂ isabout the same amplitude as I₁₀.

The relative electrode stimulation amplitudes for various photocurrentsare summarized by Table 1.

Referring to FIG. 19, a preferred embodiment of a stimulator system isshown. Stimulator system 1945 includes pulse generator and signalprocessor (PGSP unit) 1950 is connected to stimulator lead assembly1940. PGSP unit 1950 provides power to the electrodes in stimulator leadassembly 1940 and houses electronic and electro-optical components ofthe system. Stimulator lead assembly 1940 connects the stimulatorelectrodes of each stimulator lead to a controllable current source.Stimulator lead assembly 1940 further connects at least one infraredemitter to at least one optical fiber through a first fiber opticalconnector and at least one photodetector to at least one optical fiberthrough additional fiber optic connectors.

In a preferred embodiment PGSP unit 1950 is installed subcutaneously ina patient and stimulator lead assembly 1940 includes a percutaneous leador a surgical lead. In an alternate embodiment, PGSP unit 1950 isoutside the host patient's body and stimulator lead assembly includesthe percutaneous leads.

PGSP unit 1950 gathers and processes photodetector signals and makesadjustments to the stimulator electrode current (or voltage) based onthe photodetector signals. PGSP unit 1950 is connected by wirelesscommunication link 1952 across skin boundary 1956 to SCS controller1953. The SCS controller is configured to allow percutaneous activationof and adjustments to stimulator system 1945. PGSP unit 1950 is alsoconnected by wireless communication link 1955 to calibration andprogramming unit 1954. Calibration and programming unit 1954 isprogrammed to accept patient input and transmit the patient input toPGSP 1950 during calibration. In an alternate embodiment, calibrationand programming unit 1954 is incorporated into SCS controller 1953.

PGSP unit 1950 is preferably powered by batteries. In an alternateembodiment, PGSP unit 1950 derives power from capacitive or inductivecoupling devices. Wireless communication links 1952 or 1955 may furtherserve as a means of providing electrical charge for the batteries orcapacitive devices of PGSP unit 1950.

Referring to FIG. 20, a block diagram of PGSP unit 1950 is shown. PGSPunit 1950 includes CPU 2070 having onboard memory 2072 and hardwaretimer 2073. In a preferred embodiment, the memory includes two databuffers which are used as “stacks.” The hardware timer includes a timerregister. CPU 2070 is connected to pulse modulator 2062, pulse generator2060, and pulse generator 2061. Pulse modulator 2062 is connected topulse generators 2060 and 2061 which are further connected to astimulator lead through lead connectors 2083 and 2084, respectively. CPU2070 is also operatively connected to optical modulator 2068 and opticalsignal processor 2064. Optical modulator 2068 is connected to emitterdriver 2066. Emitter driver 2066 is connected to IR emitter 2079 anddrives IR emitter 2079. IR emitter 2079 includes fiber optic connector2081 to effectively couple IR emitter 2079 to a first optical fiberwhich is further connected to a first distal optical element in asurgical lead or percutaneous lead assembly.

Optical detector 2077 is connected to fiber optical connector 2082 toeffectively couple optical detector 2077 to a second optical fiber whichis further connected to a second distal optical element in a surgicallead or percutaneous lead assembly. Optical detector 2077 translatesincoming light pulses from the optical fiber into electrical signalswhich are processed by optical signal processor 2064.

In a preferred embodiment, the photodetector is similar to that of PartNo. OP501 from Optek Technology.

CPU 2070 is connected to optical signal processor 2064. Optical signalprocessor 2064 is connected to optical detector 2077 and receives anoptical signal from the photodetector and filters the optical signal.Optical signal processor 2064 may include a synchronized gated detection(e.g., lock-in amplifier type) function or other demodulation functionto improve the signal to noise ratio of the detected light.

CPU 2070 is connected to optical modulator 2068. Emitter driver 2066 isconnected to both optical modulator 2068 and CPU 2070.

In operation, CPU 2070 activates optical modulator 2068 which generatesa waveform and transmits the waveform to the emitter driver 2066. Theemitter driver then causes IR emitter 2079 to launch a light pulse withthe waveform into the first optical fiber.

The optical waveform may take several forms. For example, the pulsewidth of the optical waveform may have a low duty cycle to minimizepower consumption. A single optical pulse may occur for multipleelectrode stimulation pulses. The optical waveform may includefrequency, phase or amplitude modulation. Typical wavelength of the IRlight from the IR emitter is in a range from 800 nm to 870 nm. Typicaloutput intensity of the IR emitter is 1 to 2 mW and a suitable part isPart No. VSMY1859 from Vishay Intertechnology, Inc.

Pulse generator 2060 is connected to electrodes in stimulator leadassembly 1940 through lead connector 2083. In order to generate a pulseto the electrodes, CPU 2070 consults a calibration table stored inonboard memory 2072 to determine pulse width PW, pulse frequency Pf andpulse amplitudes for the electrodes, respectively. The pulse width andfrequency are transmitted to pulse modulator 2062 which creates amodified square wave signal. The modified square wave signal is passedto pulse generator 2060. CPU 2070 passes the amplitudes for theelectrodes to pulse generator 2060 in digital form. Pulse generator 2060then regulates the peak current or voltage of the modified square wavesaccording to the pulse amplitudes and transmits them to the electrodesthrough lead connector 2083. CPU 2070 is in transcutaneouscommunications, via RF transceiver 2071, with calibration andprogramming unit 1954 and SCS controller 1953. Pulse generator 2060 andpulse modulator 2062 may collectively be composed of a digital-to-analogconverter with associated current or voltage sources.

Pulse generator 2061 is connected to electrodes in stimulator leadassembly 1940 through lead connector 2084. In order to generate a pulseto the electrodes, CPU 2070 consults a calibration table stored inonboard memory 2072 to determine pulse width PW, pulse frequency Pf andpulse amplitudes for the electrodes, respectively. The pulse width andfrequency are transmitted to pulse modulator 2062 which creates amodified square wave signal and passes it to pulse generator 2061. CPU2070 passes the amplitudes for the electrodes to pulse generator 2061 indigital form. Pulse generator 2061 then regulates the peak amplitude ofthe modified square waves according to the pulse amplitudes andtransmits them to electrodes through lead connector 2084. CPU 2070 is intranscutaneous communications, via RF transceiver 2071, with calibrationand programming unit 1954 and SCS controller 1953. Pulse generator 2061and pulse modulator 2062 may collectively be composed of adigital-to-analog converter with associated current or voltage sources.

The modified square wave has an amplitude and duration (or width). Pulsewidths varying from 20 to 1000 microseconds have been shown to beeffective. The frequency of the pulse waveforms between 20 and 10,000hertz have been shown to be effective. The output amplitude ispreferably from 0 (zero) to +/−20 mA or 0 (zero) to +/−10 V but may varybeyond those ranges according to patient sensitivity.

PGSP unit 1950 also includes an orientation detector 2090 fordetermining the physical orientation of the patient including roll,pitch and yaw coordinates. Preferably, the orientation detector candistinguish lack of motion in the patient for a predefined period oftime. A suitable component for the orientation detector is one of partnumbers UM6-LT and MiniMU-9 orientation sensors from Pololu Corporation.

In a preferred embodiment, orientation detector 2090 is installed oraffixed to the PGSP. In a preferred embodiment, the PGSP is installed sothat the orientation detector roll axis coincides with the patient'slongitudinal axis (intersection of sagittal and coronal planes), thepitch axis coincides with a first transverse axis (intersection oftransverse and coronal planes), and the yaw axis coincides with a secondtransverse axis (intersection of transverse and sagittal planes).

Referring to FIG. 21, SCS controller 1953 is shown. SCS controller 1953includes processor 2100 connected to RF transceiver 2102, to display2104, to input/output device 2106 and to memory 2108. In the preferredembodiment, display 2104 is a low power liquid crystal display adaptedto show the current operational state of the system. I/O device 2106 isa simple push button contact array which is constantly monitored byprocessor 2100. In the preferred embodiment, RF transceiver 2102 is alow power transmitter/receiver combination.

Referring to FIG. 22, calibration and programming unit 1954 isdescribed. Calibration and programming unit 1954 includes processor 2210connected to onboard memory 2218, to input/output devices 2216 and 2217,to RF transceiver 2212 and to display 2214. Display 2214, in thepreferred embodiment, is a low power liquid crystal display.Input/output device 2216 and input/output device 2217 are simple pushbutton switches monitored continuously by the processor. RF transceiver2212 is a low power transmitter/receiver combination.

Referring to FIG. 23, the various states of the SCS controller inoperation will be described. At wait state 2305, SCS controller 1953enters a waiting posture and continually polls the I/O device andresponds to system interrupt signals, for example, a timer interrupt toenter the “run” state. Upon receipt of a “run” signal from the I/Odevice, the processor enters “run” state 2307 and transmits a “run”signal to the RF transceiver. The RF transceiver then transmits the“run” signal to PGSP 1950 for further action, for example, executing arun cycle method. After transmission, the processor returns to waitstate 2305.

While in “run” state 2307, if the patient is determined to be at restfor a predetermined period of time, then the SCS controller enters the“calibrate optics” state 2308 and the optical source is recalibrated.After the recalibration of the optical source is complete or if thepatient begins to move, the SCS controller returns to “run” state 2307.

If a “stop” signal is received from the I/O device, the processor passesa “stop” signal to the RF transceiver, which in turn sends the “stop”signal to PGSP 1950. The PGSP unit then enters stop state 2309. Theprocessor then returns to wait state 2305. If the “stop” signal includesa directive to turn off power, then power to the PGSP unit is shut downin the stop state and no electrode stimulation current is applied to thehost patient.

If a “calibrate” signal is received from I/O device 2106, processor 2100transmits a “calibrate” signal to RF transceiver 2102, which in turnsends the “calibrate” signal to PGSP 1950. The system enters “calibratestimulation” state 2311 in which parasthesia levels are optimized incertain patient positions and stimulation current calibrated for thehost patient. Processor 2100 returns to wait state 2305 aftercalibration is complete.

FIG. 24 shows calibration table 2440 for the stimulation system. Thetable is stored in memory and includes optimal electrode settings foreach patient position. In a preferred embodiment, column 2442 includesfour patient positions: forward (prone)—0°, right lateral—90°, back(supine)—180°, and left lateral—270°. Each row in calibration table 2440is associated with one of the patient positions. In an alternateembodiment, additional physical positions are included.

In the preferred embodiment, column 2444 stores values for the roll,pitch and yaw orientation for the patient. Column 2446 stores values forthe current measured for each photodetector. Column 2448 stores valuesfor the electrode stimulation pulse amplitude which produces the optimalparesthesia (or stimulation) in that patient position. Column 2450stores values for the electrode stimulation pulse width. Column 2452stores values for the electrode stimulation pulse frequency.

FIG. 25 shows an alternate preferred embodiment of a calibration table2500. In a preferred embodiment, column 2510 provides a row index.Column 2511 provides a location to store patient positions comparing tothe row index. Each row in calibration table 2500 is associated with oneof the row indices. Column 2512 stores values for the minimumphotocurrent provided by the photodetectors at the patient positions.Column 2513 stores values for the corresponding maximum photocurrentdelivered by the photodetectors. Column 2514 stores values for theminimum stimulation current amplitude for the right electrodes. Column2515 stores values for the maximum stimulation current for the rightelectrodes. Column 2516 stores the values for the stimulation currentamplitudes for the left electrodes. Column 2517 stores values for themaximum stimulation current for the left electrodes.

The maximum and minimum stimulation current amplitudes are provided toset the stimulator in a range of current amplitudes between a minimum,where no response is felt, and a maximum where the stimulation isnoxious. The maximum and minimum values are determined during electrodepulse stimulation calibration, as will be further described.

Referring to FIG. 26, an embodiment of a method of operation of thestimulation system is described. In a preferred embodiment, method 2600is implemented by a computer program which is resident in onboard memory2072 of CPU 2070 of PGSP 1950.

At step 2631, RF transceiver 2071 is polled for a change of operationcode signal received from SCS controller 1953. The system maintains itscurrent operational state until a change of operation code is received.In a preferred embodiment, a change of operation code signal isinitiated by an interrupt generated by a handware timer. In an alternatepreferred embodiment, the change of operation code can be initiated by abutton press.

At step 2633, if operation change code “run” is received, the methodmoves to step 2642. At step 2642, a stimulation routine is performed toadjust the electrode stimulation current for the patient based onphotocurrent measurements. This step is further described below.

At step 2657, the CPU determines if the patient is at rest. In thisstep, the physical orientation of the patient is monitored by readingchanges in values of roll, pitch and yaw that have occurred during apredetermined time interval. If the values are unchanged for a minimumarbitrarily defined duration, then the patient is assumed to be at restand the method moves to step 2668. At step 2668, the optical source iscalibrated based on the patient's position, as will be described morefully below. The method then returns to step 2631.

If, at step 2657, the patient is determined not to be at rest, then themethod returns to step 2631.

If, at step 2633, the operation change code is not “run”, then themethod moves to step 2635. At step 2635, the CPU determines if theoperation change code is “stop”. If the change code is “stop”, then themethod returns to step 2631.

If, at step 2633, the operation change code is not “stop”, then themethod moves to step 2637. At step 2637, the CPU determines if theoperation change code is “calibrate.” If, at step 2637, the operationchange code is not “calibrate”, then the method returns to step 2631.

If, at step 2637, the operation change code is “calibrate”, then themethod moves to step 2638. At step 2638, the CPU transmits historicaldata to the calibration and programming unit where it is stored. Thehistorical data comprises a copy of the current calibration table, avalue of optical source current, orientation sensor calibration data anda time series of electrode stimulation settings as they were performedby the stimulation routine since the previous calibration. At step 2639,the CPU performs a calibration of stimulation current levels for thepatient as will be described more fully below. The method then returnsto step 2631.

Referring to FIG. 27a , method 2700 for performing the stimulationroutine 2642 is described. The method starts at step 2742. At step 2743,a photocurrent value is measured for each photodetector with the IRemitter in the “off” state. At step 2744, CPU 2070 activates opticalmodulator 2068, which in turn activates emitter driver 2066 to generatean optical pulse from the IR emitter. At step 2746, photocurrent valuesfor each photodetector are measured with the IR emitter in the “on”state. At step 2747, the IR emitter is turned off. At step 2748,corrected photocurrent values are derived by subtracting the “off”photocurrent value from the “on” photocurrent value for each IRdetector. In a preferred embodiment, this step employs the equation:PD_(corr)=PD_(meas)−PD_(dark),   (Eq. 3)where PD_(meas) is the “on” photocurrent value, PD_(dark) is the “off”photocurrent value and PD_(corr) is a corrected photocurrent value. Thecorrected photocurrent values are stored in memory.

At step 2749, the CPU determines the electrode stimulation pulseamplitudes. In one preferred embodiment, the electrode stimulation pulseamplitudes are interpolated from the calibration table based on thephotodetector current. For example, referring to FIG. 24, if thecorrected photocurrent value of PD_(corr) has a value between PD₂ andPD₃, then a stimulation amplitude A is determined from a linearinterpolation according to:

$\begin{matrix}{A = {A_{2} = {\frac{\Delta\; A}{\Delta\;{PD}}\left( {{PD}_{corr} - {PD}_{2}} \right)}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$where ΔA=(A₃−A₂) and ΔPD=(PD₃−PD₂).

In another preferred embodiment, a spline interpolation is used. Otherinterpolation methods as known in the art can be employed.

At step 2750, the CPU optionally sets values of electrode stimulationpulse width and electrode stimulation pulse frequency. In the preferredembodiment, electrode stimulation pulse width and electrode stimulationpulse frequency are constant. In another embodiment, electrodestimulation amplitude is constant and electrode stimulation pulse widthis varied as a function of photocurrent. In another embodiment,electrode stimulation amplitude is constant and electrode stimulationpulse frequency is varied as a function of photocurrent.

At step 2752, the CPU optionally activates the pulse modulator to createa waveform which is impressed on the pulse trains sent to the electrodesand then activates the pulse generator to deliver the pulse trains. Atstep 2754, the CPU stores the corrected photocurrent values, theelectrode stimulation pulse amplitudes, the electrode stimulation pulsewidths and the electrode stimulation pulse frequencies in memory. Atstep 2755, the method returns.

Referring to FIG. 27b , alternate method 2775 of determining electrodestimulation pulse amplitudes of step 2749, is described. The methodstarts at step 2789. At step 2790, the CPU interpolates an electrodestimulation pulse amplitude from the calibration table. At step 2792,the interpolated electrode stimulation pulse amplitude is stored inmemory in a time series of interpolated stimulation amplitude values.The time series of interpolated simulation amplitude values is ahistorical record of the stimulation amplitudes applied to theelectrodes over a predetermined past period of time. At step 2796, theCPU performs a moving average over the time series of interpolatedstimulation amplitude values to determine an electrode pulse amplitude.The calculation of the electrode pulse amplitude is made using thefollowing equation:

$\begin{matrix}{{A_{ave} = \frac{{w_{k} \cdot A_{k}} + {w_{k - 1} \cdot A_{k - 1}} + {w_{k - 2} \cdot A_{k - 2}} + \ldots}{w_{k} + w_{k - 1} + w_{k - 2} + \ldots}},} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$where A_(ave) is the pulse amplitude applied to the electrode, w_(k) isa predetermined weight for value A_(k), in the time series ofstimulation amplitude values, at the current time k, value A_(k−1) atthe previous time (k−1) and so forth for earlier times (k−2), (k−3), . .. , etc. For example, the predetermined weights are predefined to falloff exponentially where w_(k)=w₀e^(−αk) and the sum in Eq. 5 is cappedto include N terms. At step 2798, the method returns.

Referring to FIG. 28a , method 2800 to calibrate the optical source ofstep 2668 is described. When the optical source degrades, generally thephotocurrent levels will decrease for a given patient orientation.Degradation of the optical source occurs for many reasons, for example,changes in the position of the surgical lead, growth of scar tissue, andfracturing of optical components and fibers, among other causes. Theoptical calibration method detects and corrects for long termdegradations in performance of the optical components of the system.

At step 2845, the method starts. At step 2852, the IR emitter is turned“on.” As a result, light from the IR emitter is reflected from thespinal cord and received by the photodetector. At step 2854, theresulting photocurrent is measured. At step 2856, the patientorientation is measured. In a preferred embodiment, the patientorientation is measured by polling the orientation detector for absoluteroll, pitch and yaw coordinates. At step 2860, the roll, pitch and yawcoordinates are then compared to those recorded in the calibrationtable. If a match is determined within a predefined confidence interval,such as ±10%, then that patient position is reported as the instantpatient position. Then the method moves to step 2862. If a match is notdetermined within the confidence interval, then the method returns atstep 2883. At step 2862, the instant photocurrent is measured andstored. At step 2864, the average photocurrent is calculated for apredetermined period of time past.

At step 2864, the average photo current value for the instant patentposition is determined. The average photocurrent value is determined fora predetermined past period of time for that patient position andreported as the average photocurrent value. At step 2870, the averageorientation and the average photocurrent values are stored.

At step 2872, an optical degradation factor is determined. In apreferred embodiment, the optical degradation is a ratio between theaverage photocurrent value and the instant photocurrent value.

At step 2876, the optical degradation factor is compared to a thresholdvalue. If at step 2876, the optical degradation factor is not more thanthe threshold value, then the method returns at step 2883. If, at step2876, the optical degradation factor is more than the threshold value,then the method moves to step 2878. At step 2878, the photocurrent valuein the calibration table for the instant patient position is multipliedby the optical degradation factor.

If, at step 2880, the optical degradation factor is greater than analert threshold, then an alert is sent at step 2882. For example, thealert can be a periodic audible sound or a displayed message on an LCDor LED display included with the SCS controller. The method then returnsat step 2883. If, at step 2880, the optical degradation factor is notgreater than an alert threshold, the method returns at step 2883.

Referring to FIG. 28b , an alternate method of calibrating the opticalsource is described. According to method 2802, the optical source isonly calibrated if the patient is in a desired position. In thepreferred embodiment, the desired position is the prone position. In theprone position, the spinal cord is farthest from the optical emitter andoptical collector of the stimulation system. Hence, the optical sourcecurrent determines the minimum detectable photocurrent level. In apreferred embodiment, method 2802 is called in step 3089 duringcalibration of the optical source.

Method 2802 starts at step 2884. At step 2885, patient position isdetermined. The patient position is determined by referencing thepatient position in the calibration table which corresponds to therunning average of corrected photocurrent value. In an alternateembodiment, the patient position is determined by polling theorientation detector. At step 2886, the patient position is compared toa desired patient position.

If, at step 2886, the patient is not in the desired position, then theoptical source is not calibrated and the method returns at step 2895.If, at step 2886, the patient is in the desired position, then themethod moves to step 2887.

At step 2887, the optical source current is stored. At step 2888, thesource current is turned “off” At step 2889, the optical source currentis turned “on” and the current to it is increased by a predeterminedamount.

At step 2890, the photocurrent from the photodetectors is measured. Atstep 2891, the photocurrent is compared to a predetermined minimumvalue. In a preferred embodiment, the predetermined minimum value isbetween 1.5 and 4.0 times the current value measured from thephotodetectors when the optical source is “off.”

If the photocurrent level is not greater than the minimum value, thenthe method returns to step 2889.

If the photocurrent is greater than the predetermined minimum value,then the method continues to step 2892. At step 2892, a final opticalsource current is set.

At step 2893, a ratio of the final optical source current to the initialoptical source current is determined. At step 2894, the photocurrentvalues in the calibration table are adjusted based on the ratio. In apreferred embodiment, all of the calibrated photocurrent values in thecalibration table are multiplied by the ratio.

Then, at step 2895, the method returns.

Referring to FIG. 29a , method 2900 for calibrating electrode pulsestimulation amplitude, at step 2639, is described.

At step 2910, the method starts. At step 2940, the orientation sensor iscalibrated. In a preferred embodiment, the orientation sensor iscalibrated to read a roll of 0°, a pitch of 0° and a yaw of 0° when thepatient is in a known position. At step 2942, the optical source iscalibrated as has been described.

At step 2950, the RF transceiver receives a signal indicative of arequest to move the patient to a prone position and passes the requestto the CPU. At step 2952, the patient is physically positioned in aprone position. At step 2954, electrode pulse stimulation amplitude isadjusted based on patient feedback to optimize the level of paresthesiaexperienced by the patient while in the prone position. This position isused to set the right and left maximum electrode pulse amplitudes. Atstep 2956, the photocurrent level and corresponding electrodestimulation pulse amplitude for the position is stored in thecalibration table.

At step 2960, the RF transceiver receives a signal indicative of arequest to move the patient to a right lateral position and passes it tothe CPU. At step 2962, the patient is positioned in a right lateralposition. At step 2964, electrode pulse stimulation amplitude isadjusted based on patient feedback to optimize the level of paresthesiaexperienced by the patient while in the right lateral position. At step2966, the photocurrent level and corresponding electrode stimulationpulse amplitude for the position is stored in the calibration table.

At step 2970, the RF transceiver receives a signal indicative of arequest to move the patient to a supine position and passes it to theCPU. At step 2972, the patient is positioned in a supine position. Atstep 2974, electrode pulse stimulation amplitude is adjusted based onpatient feedback to optimize the level of paresthesia experienced by thepatient while in the supine position. This position is used to set theright and left minimum electrode pulse amplitudes. At step 2976, thephotocurrent level and corresponding electrode stimulation pulseamplitude for the position is stored in the calibration table.

At step 2980, the RF transceiver receives a signal indicative of arequest to move the patient to a left lateral position and passes it tothe CPU. At step 2982, the patient is positioned in a left lateralposition. At step 2984, electrode pulse stimulation amplitude isadjusted based on patient feedback to optimize the level of paresthesiaexperienced by the patient while in the left lateral position.

At step 2990, the photocurrent level and corresponding electrodestimulation pulse amplitude for the position is stored in thecalibration table.

In other embodiments, the order in which the patient is positioned maybe changed. Also, additional and/or different patient positions may beadded.

At step 2991, the method returns.

Referring to FIG. 29b , alternate method 2900 for calibrating electrodepulse stimulation amplitudes, at step 2639, is described. At step 2909,the method starts.

At step 2911, the optical source is calibrated as has been described.

At step 2912, the patient is physically placed in a known position. In apreferred embodiment, the known position corresponds to one of the 0°,90°, 180° or 270° positions, previously described.

At step 2915, the minimum electrode pulse stimulation amplitude for thegiven patient position is obtained from the calibration table. In thisembodiment, the calibration table 2500 may be employed.

At step 2917, the pulse generator is directed by the CPU to send a trainof pulses to the electrodes at the minimum electrode stimulation pulseamplitude. At step 2920, paresthesia feedback is solicited from thepatient in order to determine if the level of parasthesia is optimal.

If the level of parasthesia is not optimal, then the method moves tostep 2923. At step 2923, the processor increases the electrodestimulation pulse amplitude by a discrete amount. If, at step 2924, theelectrode pulse stimulation amplitude reaches a maximum level, step 2925is performed. At step 2925, an alert is sent to the physician. The alertmay take the form of an audible sound or a text display. The method thenreturns at step 2932. If, at step 2924, the electrode pulse stimulationamplitude has not reached a maximum level, the method returns to step2917.

If, at step 2920, the level of paresthesia is optimal, then the methodmoves to step 2928. At step 2928, the optical signal processor measuresthe photocurrent for the photodetector. At step 2930, the amplitudelevels are stored in the calibration table. At step 2932, the methodreturns.

Referring to FIG. 30a , a preferred embodiment of method 3010 forperforming a stimulation routine, step 2642, is described. The methodstarts at step 3011.

At step 3014, an “off” photocurrent is measured from the photodetectorwhile the IR emitter is turned off. At step 3016, the IR emitter isturned “on.” At step 3018, an “on” photocurrent is measured. At step3020, the IR emitter is turned off.

At step 3022, a corrected photocurrent value is calculated bysubtracting the “off” photocurrent value from the “on” photocurrentvalue. At step 3024, the corrected photocurrent value is stored in thesecond data buffer. At step 3025, the oldest photocurrent value from thesecond data buffer is shifted into the first data buffer when the seconddata buffer is full.

At step 3026, a time differential value of photocurrent is determined.The time differential value is determined in order to “smooth”transitions from one stimulation value to another. In a preferredembodiment, the values of photocurrent in the first data buffer areaveraged. The values of photocurrent stored in the second data bufferare averaged. Then a difference is taken between the first average valueand the second average value according to the equation:DIFF=|PD(t ₁)−PD(t ₀)|,   (Eq. 6)where DIFF is the time differential value, PD(t₀) is the first averagevalue and PD(t₁) is the second average value.

At step 3028, a sampling rate is adjusted based on the time differentialvalue. A method for the adjusting the sampling rate is described in moredetail below.

At step 3029, the system delays for a predetermined cycle time. Thecycle time is adjusted to increase or decrease the sampling rate toconserve power when the patient is at rest.

At step 3030, the optical source is calibrated, as has been described.

At step 3035, the method returns.

Referring to FIG. 30b , method 3040 for adjusting the cycle time isdescribed. The cycle time is increased when the patient is at rest inorder to reduce power consumption. In a preferred embodiment, method3040 is called at step 3028 of method 3010.

Method 3040 starts at step 3041. At step 3042, a time differential valueis compared to the threshold value for patient movement to determine ifthe patient is moving or at rest. If the time differential value isgreater than the threshold value then it is assumed that the patient ismoving and the method moves to step 3044. At step 3044, the cycle timeis decreased so that the position of the patient is more frequentlydetermined when the patient is active.

If the time differential value is less than the threshold value then itis assumed that the patient is at rest and the method moves to step3056. At step 3056, the cycle time is increased by a predetermined cycletime increment, up to the maximum cycle time. The cycle time isincreased so that the position of the patient is less frequentlydetermined when it is changing less, when the patient is at rest. Themethod then returns at step 3059.

Referring to FIG. 30c , method 3070 is described for acceleratingcalibration of the optical source. In a preferred embodiment, method3070 is called in step 3030 of method 3010.

The method starts at step 3071. At step 3072, the time differentialvalue is compared to a predetermined threshold value. If the timedifferential value is greater than a predetermined threshold value, thenthe patient is assumed to be moving. The method moves to step 3089.

If the time differential value is less than or equal to thepredetermined threshold value, then the patient is assumed to be still.The method then moves to step 3074. At step 3074, the patient positionis determined. A running average of corrected photocurrent values,PD_(avg), is compared to the photocurrent values in the calibrationtable to determine the patient position. In an alternate embodiment, thepatient position is determined by reading the orientation detector.

At step 3075, the patient position is compared to a desired position.For example, the supine position or prone position. If the patient isnot in the desired position, then the optical source is not calibratedand the method continues at step 3089. If the patient is in the desiredposition, then the method continues with step 3076.

At step 3076, the running average of corrected photocurrent values isupdated based on the most recent corrected photocurrent value measuredfor the patient position. At step 3078, the measurement count isincremented.

At step 3080, the measurement count is compared to a predetermined countthreshold. Step 3080 ensures that the running average has been averagedover a sufficiently large number of measurements to accurately calibratethe optical source and to ensure the patient has been motionless for anadequate period. If at step 3080, the measurement count does not exceedthe predetermined count threshold, then an optical calibration cycle isnot performed. The method then returns at step 3091. If at step 3080,the measurement count exceeds or equals the calibration threshold thenthe method moves to step 3082.

At step 3082, a drift amount is calculated from the running average. Forexample, the drift amount is calculated according to:

$\begin{matrix}{{DRIFT} = {{{PD}_{avg}(S)} - \left( \frac{{P_{\min}(S)} + {P_{\max}(S)}}{2} \right)}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$where S is the row index of the calibration table of the patientposition, P_(min) (S) is from the calibration table in row S andP_(max)(S) is from the calibration table in row S.

At step 3084, the calibration table is updated. In a preferredembodiment, the sum of DRIFT and P_(min)(S) replaces P_(min)(S) in thecalibration table and the sum of DRIFT and P_(max)(S) replacesP_(max)(S) in the calibration table.

At step 3089, the optical source is calibrated as has been described. Atstep 3090, the measurement count is reset to zero. The method returns atstep 3091.

While the present disclosure has been described in terms of specificembodiments thereof, it will be understood in view of the presentdisclosure, that numerous variations upon the disclosure are now enabledto those skilled in the art, which variations yet reside within thescope of the present teaching. Accordingly, the disclosure is to bebroadly construed and limited only by the scope and spirit of the claimsnow appended hereto.

The invention claimed is:
 1. A stimulator system comprising: a controller; a first lead connected to the controller; a second lead connected to the controller; a first optical element attached to the first lead; a second optical element attached to the second lead; a first set of electrodes, attached to the first lead, operatively connected to the controller; a second set of electrodes, attached to the second lead, operatively connected to the controller; an optical source, operatively connected to the controller; an incident light beam, generated by the optical source; an optical detector, operatively connected to the controller; a photocurrent generated by the optical detector; a first optical fiber, coupling the optical source to the first optical element; a second optical fiber, coupling the optical detector to the second optical element; wherein the incident light beam is adapted to be directed by the first optical fiber to the first optical element and emitted from the first optical element to interact with an external surface; a reflected light beam adapted to be produced from the interaction of the incident light beam with the external surface; wherein the reflected light beam is adapted to be collected by the second optical element, directed by the second optical fiber to the optical detector and received by the optical detector to generate the photocurrent; wherein the controller generates and directs a set of electrode currents to the first set of electrodes and the second set of electrodes, based on the photocurrent; the controller including a processor and a memory, operatively connected to the processor; a set of programmed instructions stored in the memory; a set of calibration parameters stored in the memory; wherein the processor, when executing the set of programmed instructions, causes the controller to: determine a set of current amplitudes for the set of electrode currents based on the set of calibration parameters; store a set of historical photocurrent levels in the memory; and, derive the set of electrode current amplitudes based on the photocurrent and the set of historical photocurrent levels.
 2. The system of claim 1 wherein the set of programmed instructions further causes the controller to: derive a set of electrode current pulse widths for the set of electrode currents based on the set of historical photocurrent levels.
 3. The system of claim 1 wherein the set of programmed instructions further causes the controller to: derive a set of electrode current pulse frequencies for the set of electrode currents based on the set of historical photocurrent levels.
 4. The system of claim 1 further comprising a calibration unit, operatively connected to the controller, configured to generate the set of calibration parameters.
 5. The stimulator system of claim 1 wherein the first optical element further comprises a first mirror and a first lens and the second optical element further comprises a second mirror and a second lens.
 6. The stimulator system of claim 1 wherein the first optical element further comprises a first negative axicon and the second optical element further comprises a second negative axicon.
 7. The stimulator system of claim 1 wherein the first lead and the second lead are percutaneous leads.
 8. A stimulator system comprising: a controller; an orientation sensor operatively connected to the controller; a first lead connected to the controller; a second lead connected to the controller; a first optical element attached to the first lead; a second optical element attached to the second lead; a first set of electrodes, attached to the first lead, operatively connected to the controller; a second set of electrodes, attached to the second lead, operatively connected to the controller; an optical source, operatively connected to the controller; an incident light beam, generated by the optical source; an optical detector, operatively connected to the controller; a photocurrent generated by the optical detector; a first optical fiber, coupling the optical source to the first optical element; a second optical fiber, coupling the optical detector to the second optical element; wherein the incident light beam is adapted to be directed by the first optical fiber to the first optical element and emitted from the first optical element to interact with an external surface; a reflected light beam adapted to be produced from the interaction of the incident light beam with the external surface; wherein the reflected light beam is adapted to be collected by the second optical element, directed by the second optical fiber to the optical detector and received by the optical detector to generate the photocurrent; wherein the controller generates and directs a set of electrode currents to the first set of electrodes and the second set of electrodes, based on the photocurrent; the controller including a processor and a memory, operatively connected to the processor; a set of programmed instructions stored in the memory; a set of calibration parameters stored in the memory; wherein the processor, when executing the set of programmed instructions, causes the controller to: detect and store a set of orientation positions from the sensor in the set of calibration parameters; and, determine a set of current amplitudes for the set of electrode currents based on the set of calibration parameters.
 9. The system of claim 8 wherein the processor, when executing the set of programmed instructions, causes the controller to: detect an orientation in the set of orientation positions; detect the photocurrent for the orientation; and, adjust the set of calibration parameters based on the photocurrent.
 10. The system of claim 9 wherein the processor, when executing the set of programmed instructions, causes the controller to: determine when a patient is in a rest position based on changes in values of one or more of the group of roll, pitch, and yaw from the orientation sensor; detect the photocurrent for the rest position; and, adjust the set of calibration parameters based on the rest position and the photocurrent.
 11. The system of claim 10 wherein the processor, when executing the set of programmed instructions, causes the controller to: adjust an optical source intensity for the optical source in the set of calibration parameters based on the rest position and the photocurrent.
 12. The stimulator system of claim 8 wherein the first optical element further comprises a first mirror and a first lens and the second optical element further comprises a second mirror and a second lens.
 13. The stimulator system of claim 8 wherein the first optical element further comprises a first negative axicon and the second optical element further comprises a second negative axicon.
 14. The stimulator system of claim 8 wherein the first lead and the second lead are percutaneous leads. 